This invention relates to magnetic tags, and more specifically to magnetic tags commprising polymer encapsulated ferrite particles which are particularly useful in identifying explosives even after detonation.
Police and other investigative authorities have long been hampered in their investigations of bombing incidents by the inability to quickly and accurately determine the source of the explosive compound employed. A method to identify the explosive employed as to manufacturer, type of explosive, date of manufacture, and even the shift of manufacture would be very useful in tracing the explosive compound to its ultimate criminal user. Additionally, it is useful to identify explosive compounds used in certain mining operations to determine whether a permissible explosive compound was employed.
Such a tag for an explosive must satisfy certain requirements. First, the tag must not sensitize the explosive material, that is, the tag must not render the explosive material susceptible to premature or accidental detonation. Second, the tag must not be adversely affected by contact with the components of the explosive. Third, the tag must be able to survive exposure to the elements over prolonged periods of time without significant degradation. Fourth, the tag must be resistant to the heat and pressure of explosion. Fifth, the tag must be readily recoverable after detonation. Sixth, the procedure to identify the tag should be readily performable.
Prior art solutions to this problem have consisted of two basic approaches. The first approach employs plastic flakes approximately 0.04 millimeters wide, with each flake comprising seven colored layers with colors that are repeatable except in adjacent layers. With ten colors thus employed in seven layers, there are approximately 300 million possible color combinations (tag codes). However, these flakes have low probability of survival when employed in high energy explosives such as gelatin dynamite, boosters, and certain two-component explosives. The second approach employs ferrites having distinct Curie temperatures coated with potassium silicate or sodium silicate. However, in this latter approach, the bonding of the coating to the ferrite is weak, the coating has a tendency to deteriorate in air and the exterior of the silicate coated particles is not sufficiently smooth for use in tagging explosives. Both of these prior art tagging approaches utilize a phosphor to facilitate post-detonation detection and retrieval of the tag particles. Nevertheless, both of these approaches exhibit undesirable survival characteristics.